Radial, or centrifugal compressors, are constructed in such a manner as to convert a fluid velocity pressure to a static pressure and thereby provide a compression of the fluid. Rotating impellers are effective to impart a velocity pressure to the fluid and deliver the same to a diffuser section where the velocity pressure is converted to a static pressure. The use of fixed vanes in the diffuser section results in improved compressor efficiency, generally at the expense of operating range. Conversely, a vaneless diffuser will yield a wider operating range, but will not achieve a performance level as high as the vaned design. It is well known that the provision of stator vanes in a compressor diffuser section allow velocity pressure recovery of up to three times that of a vaneless diffuser.
Most users require centrifugal compressors which are adapted for operation over a variety of flow conditions and over various ranges. However, there is typically one flow condition at which the compressor most frequent operates. Many users therefore prefer the best possible performance at a "design" condition, but still require some degree of "off-design" performance. Often, this is not possible or practical, as in compressors using vaned diffusers, the performance degrades during "off-design" conditions. This degradation is principally a result of increased losses due to flow incidence of the fluid on the diffuser vanes. Incidence is generally defined as the angle of the vane with respect to the direction of fluid flow around it. If it were possible to partially or totally retract the vanes from a diffuser passage, the losses due to incidence would be relieved, and the flow range would increase.
Prior attempts to address the problem of vane incidence and its resultant effects on flow range fall into two known groups. In the first, alteration of fluid flow angle is accomplished by reducing the diffuser passage width, changing the fluid flow direction accordingly, thus reducing the incidence levels. This approach is shown in U.S. Pat. No. 3,365,120, issued Jan. 23, 1968 to Jassniker. The second approach is to rotate, or otherwise change the orientation of the diffuser vanes to match the fluid flow angle, thus reducing the incidence losses. The second approach suffers certain deficiencies in that it is difficult to accurately pivot all of the vanes to the same angle, primarily due to backlash and mechanical play, thus resulting in flow turbulence. This latter approach is disclosed in a product brochure, "CVM Centrifugal Compressors," 1987, by Dresser-Rand.